As you have Windows Home Premium 64 bit, definitely place some more RAM into your system!
Overclock your Q6600! Get an after market CPU cooler like COOLER MASTER Hyper 212 EVO http://www.newegg.com/Product/Product.aspx?Item=N82E16835103099
As you have a 850W power supply, try to find a second hand GTX 260 to make a 2x SLI.
Get yourself a SSD to improve performance, and use your 500GB HD as storage drive.